Several methods exist for accessing a plurality of telephone trunk lines, termed central office (CO) lines, within or by a single telephone set (station). The oldest and most commonly used method is an electromechanical key telephone in which several CO lines are brought into a single station, connection between the CO lines and the telephone apparatus being made mechanically through a switch. In a typical key telephone, several CO lines are brought in parallel into a single telephone station, and the station is equipped with keys and switches in order to connect each of the CO lines, one line at a time, with a single telephone apparatus.
Because several stations typically must have access to the same CO lines, a key telephone system includes a distribution box, termed a key service unit (KSU), which distribute the tip and ring wires from each CO line, in parallel, into the several telephone stations. When several stations share several CO lines in this manner, visual indicators in the keys (or adjacent to them) are commonly used for signaling purposes to identify, for example, which CO line is ringing, which CO line is engaged, or which line is on hold. Furthermore, additional circuits, such as intercom paths, can be provided in parallel to the CO telephone lines, utilizing parts of the same equipment for local communication, but involving separate keys for intercom signaling purposes.
From the above description it is clear that every CO line which is distributed from the KSU, is physically connected to the terminal station telephone by its own tip and ring wires. The wiring cable in an electromechanical key system, which interfaces each telephone station with the KSU, must include at least one pair of wires for each CO line. Additional wires are used to transmit the visual signals, and to control the hold circuits shared by all the telephone stations. Because it is desirable to have an indication (to all users) as to which CO line is engaged at a particular moment, a particular set of wires is used to send such signals from the mechanical keys into the circuits of the KSU. It is necessary therefore, to use multiwire cables to interface the telephone sets with the KSU; and the key telephone cable and the KSU cable are commonly connected by multipin plugs and jacks.
While electromechanical key systems are simple wire distribution networks which permit manual CO line selection, they have inherent limitations that arise because, in such systems, the CO lines are not only physically separated, but their connections to all stations in which they terminate are equipotential. For example, a call coming from the outside will reach all stations connected to the CO line on which the call comes in, and can be picked up by any or all such stations. Such outside calls cannot be directed, selectively, into a specific station if the CO line is distributed to other stations as well. Furthermore, since the tip and ring pair of each CO line is totally separated from other pairs, signaling and intercom between stations is possible only through specialized additional circuits and wires. The physical separation of the CO lines in both the KSU and the telephone sets does not permit the utilization of conventional telephonic accessory equipment and devices (e.g., answering machines) on more than one CO line per device, thus making such additions quite expensive.
The simplicity is manifested also by the inability of the currently available key telephone sets to provide special features, even such conventional features that exist in single-set household telephones (e.g., abbreviated dialing and redialing); and if it desired to add to such functions, the telephone sets must be modified. Thus, in order to access specific dialing routes serviced by long distance carriers, for example, users have to either manually dial complex and lengthy access numbers and codes of the carrier switch, or use dedicated lines. The lack of simultaneous central control of the several CO lines in the KSU, limits monitoring of telephonic traffic data (e.g., numbers called, time of call, etc.), and impose difficulties on attempts to restrict access to toll calls. These limitations can be overcome by restructuring the KSU or by replacing the telephone sets and wires with specialized sets; but the procedure is both expensive and complicated.
Finally the lack of central control limits the application of secretarial functions (e.g., reminders, messages, etc.) and requires human intercession in order to carry out such functions. However, for conventional dialing out, and for answering a call, the electromechanical key system is simple and easy to operate; and such system costs much less than other, more advanced systems.
An alternative telephone system, with which it is possible to overcome some or all of the above mentioned limitations, is the electronic key system in which each telephone apparatus is accessed by only one pair of tip and ring wires. Switching between the various CO lines and the telephone stations is done in a central, electrically or electronically controlled, switching matrix located in an EKSU (Electronic Key Service Unit). Typically, the telephone station contains keys which enable the user to identify to the switching matrix which CO line the user wishes to have connected to the station. Such keys are used in a similar manner to that of the former system, in that one key is used for each CO line. Signaling and visual status indicators are made available by additional one or two pair of wires, which carry the necessary electronic signals to perform the CO line selection and control, and to turn on and off the visual indicators.
The availability of a centralized switching matrix permits a higher level of functions than can be achieved in electromechanical key telephone systems. In the more advanced electronic key systems, a microprocessor controls the switching matrix and enables central computerized signaling between different stations, indication of CO line status, transfer of calls from one station to another, and sharing of modern features such as speed dialing and redialing between several telephone stations.
While the one-to-one relation between CO lines and stations (in which particular CO lines are accessed by selected keys in the telephone station) is common to both electronic and electromechanical systems, another switching system, which employs a dynamic mode of connection, is the PBX. In a PBX, which is a miniaturization of a central telephone office, a group of telephone stations is accessed by a group of CO lines. Each station can be connected to each of the CO lines, not by a user selection, but rather by a programmed access of the central switching matrix of the PBX which controls the connections between lines and stations according to their availability and without active user involvement. As a result, the user is unaware which of the lines he is using. In order to connect to a CO line, a user must dial a request code to the PBX; and one of the CO lines out of those available is then connected to his station.
In order to transfer an incoming call to a particular station, the PBX uses the same dialing principles used in a central office. Each station can be accessed by dialing its directory number to the PBX not only from the outside, but also by other stations making interconnection possible. Since all CO lines are under central control, the PBX permits automatic, and easier routing of calls from particular stations served by it through particular lines (for example, low cost lines), as well as easier and more efficient methods of distributing incoming calls to different stations, and monitoring and recording data associated with incoming and outgoing calls. These functions provide a high degree of management and control of the telephonic traffic, and help to increase work efficiency and reduce telephone use expenses, by programming communication schedules and pathways.
In the case of a PBX system, the user can access a particular PBX function with telephone stations that need not be equipped with special function keys. All functions performed by the PBX can be accessed by dialing special codes from the conventional dial pad of a conventional telephone set because the central PBX unit is equipped with a decoding mechanism that translates the codes into functions. However, while electronic key systems and PBX's offer more functional advantages, their use requires more effort and their cost is higher than that of simpler systems.
The three types of telephone switching systems described above are not only distinctly different, but are also non-compatible in many cases. It is desirable, if possible, to utilize the functional advantages and cost benefits of each and combine them. It is, therefore, an object of the present invention to provide apparatus by which certain advantages typically available heretofore, mainly in PBX system, but some of which are found also in electronic key systems, can be added to existing or to new electromechanical system without any change of the various components of the key system, and at relatively low cost.